What is landscape photography, in this day and age? Monuments of natural beauty, frozen in perfect light? Or the opposite of that, scenes of desecration, revealing how we have mauled and mutilated our inheritance? Or, given the vast potential opened up by digital photography, has it become a visual offshoot of fantasy or science fiction?

In an exhibition of exuberant generosity and breadth, curator William Ewing's answer is yes to all of the above, and anything else that might claim a place and that his magpie curiosity alights on. The east wing of Somerset House is not a big venue, but this is a huge show in every sense, of a scale which does justice to its planetary subject matter and goes far beyond that, to explore the boundaries of space. One of the many remarkable pictures here shows the Earth snapped from four billion miles away as a tiny white dot – only visible at all because the Pacific Ocean was turned towards the sun when the shutter was released. It's phantasmagorical.

We still have the glories of nature: thunderous glaciers by Olaf Otto Becker, Simon Roberts's gorgeous English panoramas, with beach walkers at Padstow and hang gliders over Sussex, and ironic urges held firmly in check. Elger Esser's lemon-yellow river fringed by palms looks like a sepia Victorian shot of the Nile brought up to date, though in fact it is a river in California. From a great height Harry Cory Wright points his 10x8 Gandolfi at the English Channel and captures both its pelagic scale and the quicksilver beauty of the light on it.

But the age of innocence is long gone and the ways in which we have ripped and butchered nature are legion. The most striking of the works here discover a macabre beauty in the damage: Edward Burtynsky's river running red with nickel tailings, David Maisel's visions of the garish dried-out lakes north of Los Angeles, spewing chrome and cadmium dust into the atmosphere, James Stillings's ruined corner of the Amazon rainforest rendered almost as an abstract oil painting, Daniel Beltra's pellucid picture of an oil spill. And lest we are tempted to take the moral high ground when looking at these pictures, David Maisel explained to journalists at a preview last week that one of the factors that fuelled his desire to photograph toxic extraction sites was the fact that this is where some of the chemicals still required for the creation of photos like this are sourced.

The primary division of the show is into Fact, happy or hideous, and Fiction, this section acknowledging the degree of control which present-day photographers have over the final image through Photoshop and its more sophisticated equivalents. But only a minority of the pictures in Fiction fit that description, and those are not the most interesting: a chain of mountains digitally rearranged so their profile describes the ups and downs of the stock market, Marcus Lyon's aerial picture of hundreds of containers in a Chinese port, digitally orchestrated into "a modern stained-glass window".

There is nothing particularly fictive about most of the other works in Fiction, but they are a lot of fun: notably an entire wall of sunset snaps downloaded from among the 12 million such pictures uploaded by amateurs on to Flickr; the shot of Earth from 4 billion miles away; and the poignant final picture by Liu Xiaofang, entitled I remember, of a Chinese girl with her hair in bunches, perched on the edge of a cliff and gazing at a cloud.

Some of the most radiant works here owe a clear debt to past masters of landscape such as Ansel Adams, and one could have done with more of that: although Thomas Struth is represented, there is nothing by some of the great photographers he influenced, such as Massimo Vitali and Domingo Milella and the impressive English nature photographer Jem Southam. But with photography in such a centrifugal state, Ewing cannot be faulted for throwing his net as wide as possible – as far as the planet Mars and beyond. The result is always exhilarating.